There have recently been increasing demands that substrates formed of organic materials such as plastics and inorganic materials such as glass be improved in terms of antifogging properties and antifouling properties.
To solve the fogging problems, a method has been proposed in which an antifogging coating containing a reactive surfactant and an acrylic oligomer is applied to provide enhanced hydrophilicity and water absorption (see, for example, Non Patent Document 1). The fouling problems have been addressed by methods in which the hydrophilicity of the surface of materials is enhanced so that fouling such as airborne hydrophobic substances which have become attached to surfaces such as exterior walls can be detached and removed from the surfaces by water spray or rainfall (see, for example, Non Patent Documents 2 and 3).
Further, hydrophilic materials have been proposed in which a crosslinkable polymerizable monomer composition is applied onto the surface of a substrate and is incompletely polymerized while controlling the dose of UV irradiation to form a crosslinked polymer, and subsequently a hydrophilic monomer is applied and UV rays are applied again to block- or graft-polymerize the hydrophilic monomer to the surface of the crosslinked polymer (Patent Document 1 and Patent Document 2).
However, this simple block or graft polymerization of a hydrophilic monomer to the surface of a substrate only attaches the hydrophilic groups to the surface, resulting in low durability. Thus, the hydrophilic materials have drawbacks in that they cannot withstand long-term use.
Further, there have been presented hydrophilic materials which include a compound having an anionic hydrophilic group and an ethanolamine (Patent Document 5), and hydrophilic materials which include a compound having a non-neutralized anionic hydrophilic group and an inorganic colloid sol (Patent Document 6). However, the incorporation of an ethanolamine causes problems such as sticky surface due to the ethanolamine per se remaining inside the film, and poor affinity for water. The use of a compound having a non-neutralized anionic hydrophilic group results in problems such as that the film will not attain sufficient hydrophilicity and will have low transparency.
To solve the above problem, the present inventors have proposed a monolayer film in which specific anionic hydrophilic groups have a concentration gradient (an uneven distribution) between the inside of the film and the film surface and the anionic hydrophilic groups are present in a higher concentration near the surface (Patent Document 3 and Patent Document 4).
Films obtained by such methods attain excellent hydrophilicity and transparency. However, a further enhancement in transparency is often demanded in applications where extremely high transparency is required, for example, in optical components.